Allow relative image urls embedded in Text items to be resolved against a specified base URL.

Currently embedded links in rich text are resolved against the URL of the text item. Relative links in text loaded from an outside source are likely to be relative to that source though not the item so it should be possible to specify that source URL so the links can be resolved correctly.

Something like:

Text {
    text: reader.text   // some object that reads text from a file/network.
    baseUrl: reader.source
}